A conventional welding torch generally includes a cable assembly connected to a torch body, a gooseneck extending from the body, and a torch head at a distal end of the gooseneck. The torch head typically includes a retaining head and/or diffuser, a contact tip, and a nozzle. Welding wire (consumable electrode) and shielding gas are fed through the cable assembly and gooseneck to the torch head, where the welding wire and shielding gas are fed out of the contact tip and nozzle.
Common metal welding techniques employ heat generated by electrical arcing to transition a portion of a workpiece to a molten state, and the addition of filler metal from the welding wire. Energy (e.g., welding current) is transferred from the cable assembly and gooseneck through the front components of the torch including the retaining head and contact tip, to the consumable electrode welding wire. When a trigger on the welding torch is operated or an “on” signal is assigned by a robot/automatic controller, electrode wire is advanced toward the contact tip, at which point current is conducted from the contact tip into the exiting welding wire. A current arc forms between the electrode wire and the workpiece, completing a circuit and generating sufficient heat to melt the electrode wire to weld the workpiece. The shielding gas helps generate the arc and protects the weld. As the electrode wire is consumed and becomes a part of the weld, new electrode wire is advanced, continuously replacing the consumed electrode wire and maintaining the welding arc.
In order to increase welding speeds (e.g., the travelling speed) and to reduce spatter generation in welding applications, welding power sources have been utilizing modern waveforms that are represented by pulse and controlled short circuit. As shown in FIG. 1, these waveforms typically use high peak current (I_Peak) in a short pulse period and high current ramp rate. For example, 300 amps is usually regarded as a high current (I_CV) for 1.13 mm (0.045 inch) outer diameter (OD) solid steel electrode wire in constant voltage welding applications. In contrast, in pulse welding applications it is common for this same electrode wire to be welded at a peak current of 500 amps. This 67% higher current results in 178% more heat generation (in joules) at the contact tip—electrode wire interface, according to the rule E=I2Rt where E represents heat in joules, I represents the current, R represents the electric resistance across the contact tip—electrode wire interface, and t represents a duration of time.
The high welding current and high current ramp rate transferring across the contact tip—electrode wire interface during pulse welding applications causes local melt or evaporation (e.g., arc erosion) on both the electrode wire and the contact tip. For example, burn marks form on the electrode wire as it is fed through the contact tip. This pattern of burn marks on the electrode wire is a characteristic feature of modern pulse waveform welding and is not seen on electrode wire fed through contact tips during constant voltage welding modes. It is also postulated that similar damage occurs on the inner surface of the contact tip where welding current is transferred to the electrode wire. However, burn marks cannot be or are difficult to observe on the inner surface of a used contact tip due to tribological wear of the burn features by the feeding of the electrode wire against the inner surface of the contact tip.
Arc erosion during pulse welding applications causes substantial wear removal of the contact tip, and practical data indicates that contact tips deteriorate faster in pulse welding applications in comparison to constant voltage applications. For example, the length of a wear mark on an inner surface of a contact tip used for 20 minutes in a constant voltage application was measured at approximately 3 mm. Using the same pack of electrode wire and the same wire feeding speed, the length of a wear mark of a contact tip used for minutes in a pulse application was measured at approximately 11 mm. After two hours of pulse welding, the wear mark on the same contact tip measured 17.5 mm. Thus, it is apparent that pulse welding can cause significantly more contact tip wear than constant voltage welding.
As a contact tip is used and deteriorated, the energy transfer efficiency between the contact tip and the electrode wire decreases. This results in lower energy consumption at the arc. When the energy consumption is too low to maintain a smooth welding arc, stubbing occurs, which causes undesired welding defects such as cold welding and discontinuous beads.
Further, electrode wire always has an inherent cast, or curvature, due to the packaging of the electrode wire and the fact that the electrode wire if fed through a curved welding torch. The curved electrode wire is bent (elastically or plastically) inside the contact tip when it is forced into the central hole that extends through the contact tip. The electrode wire is typically bent against one “side” of the contact tip hole at a front end (called the front contact point) of the contact tip and at an opposite “side” of the contact tip hole at a rear end (called the rear contact point) of the contact tip. This mechanical bend is essential for the contacting force and ensures electrical conduction between the electrode wire and the contact tip.
FIG. 2 illustrates the contour of a section of electrode wire 20 inside a used (worn) contact tip 22. As the contact tip 22 is used and worn, the front portion 24 of the contact tip hole is damaged (e.g., keyholed when viewed from the front end), and the contact point 26 becomes a contact area 28. The welding current, which in a new contact tip is delivered to the electrode wire at the very front end of the bore 24, is now transferred across the contact area 28 (as depicted by arrow 30). This increases the total electric resistance of the welding circuit and causes low welding current, or low efficiency of the contact tip.
One method that has been used to mitigate the deterioration of the contact tip is to increase the mechanical contact force between the contact tip and the electrode wire, such as by spring mechanisms, S-shaped contour of the bore of the contact tip, or introduction of more curvature to the electrode wire before feeding it into the contact tip. Improving the contact force reduces the electrical resistance and the fluctuation of electrical resistance across the interface, thus improving the contact tip efficiency. However, these designs are either too expensive to be commercialized, or too fragile to tolerate the harsh nature of the welding environment, such as high temperatures and spatter.